[textures] ImageDraw() and ExportImage() (.png) does not support f32bit per channel image data

[rmn20]

• I tested it on latest raylib version from master branch
• I checked there is no similar issue already reported
• My code has no errors or misuse of raylib

Issue description

I created PIXELFORMAT_UNCOMPRESSED_R32 format images, tried to combine them into one large image and export, but for some reason the image file was empty and rendering was broken. I replaced ImageDraw with memcpy and it helped fix the rendering with texture created from these images, but export was still broken.

Environment

Windows 8.1 64x
ATI Technologies Inc.
AMD Radeon HD 8330
GLSL 4.60

[raysan5]

@rmn20 You are right, ImageDraw() does not support f32bit format images at this moment.

Afaik, PNG only supports up to 16bit per channel but stb_image_export only supports up to 8bit per channel image export.