readme: add some basic description and instructions

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# Natural display engine with TeX syntax for fx-9860G and fx-CG 50
This library is a customizable 2D math rendering tool for calculators. It can be used to render 2D formulae, either from an existing structure or TeX syntax.
```latex
\frac{x_7}{\left\{\frac{\frac{2}{3}}{27}\right\}^2}
```
![Sample on a Graph 35+E II](https://framapic.org/CkFr4ab3FjcZ/bXQg9LyozGqg.jpg)
## Features
* Build formulae from TeX syntax
* Highly-customizable graphical and layout parameters
* Drawing is based on a few user-provided primitives (point, line, text size,
text)
* Compatible with fxlib and gint
List of currently supported elements:
* Fractions (`\frac`)
* Subscripts and superscripts (`_` and `^`)
* Grouping parentheses, brackets and braces (`\left` and `\right`)
Features that are partially implemented (and what is left to finish them):
* Support for inline style and display style (compute display-style subscript
and superscript layout, and expose a parameter)
* API functions to build TeX objects without a TeX formula (expose functions
which are currently in the parser)
* Full fx-CG 50 support (suitable parenthesis styles)
See the `TODO.md` file for more features to come.
## Building
First specify the platform you want to use :
* `cli` is for command-line tests, with no visualization (PC)
* `sdl2` is an SDL interface with visualization (font support coming) (PC)
* `fx9860g` builds the library for fx-9860G targets (calculator)
* `fxcg50` builds the library for fx-CG 50 targets (calculator)
For calculator platforms, you can use `--toolchain` to specify a different toolchain than the default `sh3eb` and `sh4eb`. The install directory of the library is guessed by asking the compiler, you can override it with `--prefix`.
Example for an SDL setup:
```sh
% ./configure --platform=sdl2
```
Then you can make the program, and if it's a calculator library, install it. You can later delete `Makefile.cfg` to reset the configuration, or just reconfigure as needed.
```sh
% make
% make install # fx9860g and fxcg50 only
```
## Using in a program
Before using the library in a program, a configuration step is needed. The library does not have drawing functions and instead requires that you provide some, namely:
* Draw a pixel (`TeX_intf_pixel`)
* Draw a line (`TeX_intf_line`)
* Compute the size occupied by a string (`TeX_intf_size`)
* Render a string (`TeX_intf_text`)
The three rendering functions are available in fxlib; for monospaced fonts the fourth can be implemented trivially. In gint, the four can be defined as wrappers for `dpixel()`, `dline()`, `dsize()` and `dtext()`.
The type of formulae is `TeX_Flow`. To parse and compute the size of a formula, use the `TeX_parse()` functions, which returns a new formula object (or `NULL` if a critical error occurs).
```c
char *code = "\\frac{x_7}{\\left\\{\\frac{\\frac{2}{3}}{27}\\right\\}^2}";
struct TeX_Flow *formula = TeX_parse(code);
```
The size of the formula can be queried through `formula->width` and `formula->height`. To render, specify the location of the top-left corner and the drawing color (which will be passed to all primitives):
```c
TeX_draw(formula, 0, 0, BLACK);
```
The same formula can be drawn several times. When it is no longer needed, free it with `TeX_free()`:
```c
TeX_free(formula);
```

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* Write real error messages
* Support Unicode symbols (probably requires help from the font side, urgh)
* Don't use TEX_LAYOUT_SPACING for everything, just make it the default
* Add a parameter to resolve as inline style or display style
* Add sum and products
* Add square roots, and honor TEX_SQRT_SLANTED and TEX_SQRT_BAR_LENGTH

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@ -121,7 +121,6 @@ int main(void)
struct TeX_Flow *flow = TeX_parse(formula);
if(!flow) { puts("parsing error!"); return 1; }
size_flow(flow);
TeX_debug_flow(flow, 0);
SDL_SetRenderDrawColor(r, 0xff, 0xff, 0xff, 0xff);